A high pulse energy system based on a Master Oscillator Power Amplifier often requires a reduction of the repetition rate of the oscillator to a desired user frequency via a pulse picking mechanism. The pulse picking is typically accomplished by using a pair of Pockels cells, which are electro-optic devices that cause a polarization retardation to a laser beam when a high voltage (several kV) is applied. For the formation of short duration pulses, the high voltage signal to the electro-optic crystal must be turned on and off very rapidly. The rise time can be adjusted to be on the order of 3-5 nanoseconds, but to keep the fall time on the pulse on the same duration necessitates sometimes the use of a second Pockels cell. Using Pockels cells, the pulse picking repetition rate can be adjusted to be around 1 MHz.
Although effective, this method of pulse picking is not used here for multiple reasons. First, a pair of high-speed Pockels cells and associated high-speed/high-voltage electronics is not readily available as an ‘off-the-shelf’ system and must be specially designed and built. Not only these devices are expensive, but in addition, the use of high voltage electronics raises safety and reliability concerns.
An alternative method of creating a burst of pulses from a high repetition rate oscillator output is through the use of an acousto-optic modulator (AOM). AOMs, also commonly referred to as Bragg cells, operate on the principles of the acousto-optic effect, where an acoustic wave traveling through a crystal or liquid causes a small variation in the index of refraction. This variation appears to an optical beam passing through the medium as a sinusoidal grating with a wavelength equal to the acoustic wavelength. Thus, the incident light will be diffracted in the presence of the acoustic wave. Depending on the optical and acoustic properties of the AO material two operating regimes can occur: either isotropic or anisotropic. Isotropic interaction does not change the polarization of the beam with the AOM's being operated in Raman-Nath or Bragg regime, where most of the incident light can be diffracted into the first order. Anisotropic AO interactions change the polarization of the optical beam, and they result in a single diffracted order, with a higher efficiency and a larger acoustic optical bandwidths than the isotropic AO interactions.
Several properties of the AOM devices are important for pulse picking applications. Diffraction efficiency of the AOM device represents a percentage of how much the incident light is diffracted in the first order. A good conversion efficiency can account for as much as 90% of the zero order beam power into the first order beam power for an input beam with low divergence. A typical diffraction efficiency is around 80%. When a focused beam is used into the AOM, the diffraction efficiency tends to be lower due to the mismatch divergences of the optical and acoustic waves. Furthermore, there is an increased risk associated with focusing of the incident beam which can cause optical damage for even relatively low laser power levels. However, the faster response of this type of AOM is a convenient and useful characteristic that makes the focused type AOM commonly used in pulse picking setups.
The response time is determined by the time necessary for the acoustic wave to pass the optical beam. Besides the beam spot size in the AOM, the response of the AOM RF driver and the incident laser beam profile can further affect the overall response time of the AOM.
For a beam with waist diameter D, this time is given by the ratio of the beam diameter to the speed of sound in the material v.
            t      r        =                  0.65        ⁢                                  ⁢        D            v        ,
Different materials can be selected for the use in the AOM devices, depending on the particular desired rise time value and the input power. Typical values for the speed of sound are 4.21 mm/μsec (TeO2), =3.63 mm/μsec (PbMoO4), and 5.96 mm/μsec (Fused Silica).
The ability of separating a single pulse at a desired frequency is a crucial factor in pulse picking applications. In order to successfully isolate a single pulse from the oscillator pulse train, a faster rise time of the AOM becomes necessary, and therefore a smaller spot size inside the AOM should be used. Utilizing a beam with a small diameter inside the AOM introduces a series of limitations and drawbacks. Due to the angular dependence of the diffraction efficiency on the light propagation direction, when a beam is focused too tight inside the AOM, the diffraction efficiency decreases, as mentioned before and the beam profile can become distorted.
When a single pulse at a lower repetition rate is selected, the pulse energy is small, and needs to undergo further amplification. Any amount of light passing through the AOM when the applied RF frequency is off—which is called background light, will be amplified in the downstream optics and reduce the gain for the useful pulse picked signal. Minimization of the background light can be done by appropriately choosing the extinction ratio of RF driver, which is the ratio between the maximum output RF level to the minimum output level. Typically, most RF drivers can achieve extinction ratios of 40-45 dB, which is not enough for some applications. Higher RF extinction ratios are difficult to achieve, and become an expensive solution for reducing the background light level associated with this poor extinction.
The background contrast ratio is not the only parameter that needs to be optimized in pulse picking setups. Due to the response time of the AOM, a small prepulse preceding the main pulse and a small post pulse following the main pulse can appear. The side pulse contrast ratio represents the ratio between the peak intensity of a main pulse and the intensity of any low lying pre-pulse or post pulse and its value needs to be minimized for efficient amplification in further stages.
To lower these contrast ratios, the use of two AOMs in series has been proposed to clean up the background pulses and to improve the side pulse contrast ratio. Several commercial products based on this idea are offered for improving the contrast ratio in the AOM. The high contrast ratio (6000:1) is achieved by placing two Bragg cells in a series configuration, with one beam passing through two Bragg cells. The optical setup becomes a challenging task for achieving a good pulse picked beam profile from the AOM devices due to the elipticity of the diffracted order. Correcting this elipticity often requires additional components such as cylindrical lenses and a careful optical alignment procedure. Therefore, this approach requires not only multiple Bragg cells which are driven by additional RF drivers, but additional optical components which offsets the practical advantages of the AOM pulse picker versus the EO based pulse pickers. See U.S. Pat. No. 7,907,334 B2
Furthermore, the amplification of the pulse picked beam in further stages has to take into account a proper isolation system that will prevent damaging the device due to the self lasing or back reflections. There is still a need for a simple, compact and reliable AOM pulse picker system with a low component cost that will preserve the excellent background and prepulse contrast ratio f the EO devices.